Method for detecting photocopied or laser-printed documents

ABSTRACT

A method for distinguishing photocopied or laser-printed documents from original documents produced by offset printing, handwriting, or typewriting. A document is scanned at low-resolution and at high-resolution to produce a low-resolution and a high-resolution matrix representation of the presence or absence of ink or toner at discrete locations on the surface of the document. Printed regions detected at low-resolution are used to mask regions of the high-resolution matrix representation from the analysis. The remaining unmasked regions of the high-resolution matrix representation are analyzed to detect discrete microdots uniformly distributed within those regions. The presence of microdots on the surface of the document indicates that the document was produced as a photocopied or a laser-printed duplicate.

TECHNICAL FIELD

The present invention relates to the authentication of printed documentsand, in particular, to a method for distinguishing original printeddocuments from photocopied or laser-printed duplicates of printeddocuments.

BACKGROUND OF THE INVENTION

The large variety of different types of paper documents used incommercial, governmental, and private transactions can be broadlyclassified into two categories. In the first category are originaldocuments produced by offset printing, handwriting, typewriting, ink jetprinters, dot matrix or other impact printers, thermal and thermaltransfer printers, pen-based plotters, and other such methods. Thesecond category of paper documents includes duplicates or copies oforiginal documents produced by photocopy machines or laser printers.

Photocopying and laser printing technologies have greatly improved inquality and capabilities in recent years. High-resolution color printersare, for example, of sufficient quality to be used by paper currencycounterfeiters. Because monetary values and legal rights are frequentlyassociated with original documents, but not with copies or duplicates ofthose documents, it is very important for those holding or receivingtransactional documents to be able to distinguish original documentsfrom copies of original documents. However, because of technologicaladvances in photocopying and laser printing, it is becoming increasinglydifficult for these holders and receivers of documents to distinguishoriginal documents from copies.

FIG. 1 introduces an example of the problem of distinguishing anoriginal document from a copy that will be used below in the DetailedDescription of the Invention. FIG. 1 shows an original bill of sale 102and a photocopy of the original bill of sale 104 produced by a highquality, high-resolution color printer. To the naked eye, there is noperceptible difference between the original bill of sale 102 and thecopy 104.

There are many reasons why a holder or receiver of the bill of salemight wish to distinguish the original bill of sale from a copy. Aholder of a bill of sale might, for example, be able to present the billof sale to the seller in order to receive a rebate, a discount on asubsequent purchase, or a refund for damaged or faulty goods. It isimportant for a seller or merchant to be able to distinguish theoriginal bill of sale from a copy to avoid paying out multiple refunds,discounts, or rebates for a single purchase. The holder of a bill ofsale may wish to obtain some further document or authentication based onthe original bill of sale. For example, the purchaser of a car may wishto obtain title to a car by presenting an original bill of sale to agovernment office. Presenting a copy or duplicate bill of sale maypossibly constitute fraud. The holder of the bill of sale may wish toauthenticate the bill of sale to avoid possibly committing fraud, andthe receiving government office may wish to authenticate the bill ofsale in order to avoid licensing the car to someone other than theowner.

Besides authentication, there are other reasons for which it may beuseful to distinguish laser-printed or photocopied documents fromoriginal documents. For example, a laser printer or photocopy machinemight send documents that have been previously laser-printed orphotocopied through a different paper path than an original document inorder to avoid jamming. A need has therefore been recognized for amethod for faithfully and automatically distinguishing an originaldocument from a photocopied or laser-printed document.

SUMMARY OF THE INVENTION

The present invention provides a method for distinguishing laser-printedor photocopied documents from original documents provided by offsetprinting, handwriting, typewriting, and other such methods. In oneembodiment of the present invention, a document is scanned with anoptical scanner at low-resolution and again at high-resolution. Opticalscanning provides a two-dimensional matrix, or grid, in which each cellor position corresponds to a physical position on the surface of thedocument and each cell contains one of two values that indicate whetherink or toner was detected by the scanner at that position on thedocument. Those sections of the high-resolution matrix that correspondto blank or white space in the low-resolution matrix are then analyzedto detect small, randomly-spaced microdots that appear only athigh-resolution. These small, randomly-spaced microdots are present onphotocopied or laser-printed documents, but not on original documents.Those documents that exhibit small, randomly-spaced microdots are thusidentified as photocopied, or laser-printed documents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an original bill of sale and a photocopy of the originalbill of sale.

FIG. 2 shows the types of imperfections that can be detected in originaldocuments and in copies of original documents at high-resolution.

FIG. 3 is a block diagram showing the components of a system on whichthe present invention may be implemented.

FIG. 4 shows a portion of the example document suitable for analysis.

FIG. 5 shows a grid representing a low-resolution optical scansuperimposed over a portion of the region shown in FIG. 4.

FIG. 6 shows a high-resolution scan of the same region scanned atlow-resolution in FIG. 5.

FIG. 7 shows idealized microdots positioned within a portion of ahigh-resolution scan grid.

FIG. 8 is a flow control diagram for one embodiment of the presentmethod.

FIG. 9 illustrates operation of the routine Photocopy.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for determining whether adocument has been copied or duplicated by a photocopier or a laserprinter. In one embodiment of the present invention, a document isoptically scanned at a low-resolution and again at a high-resolution. Inthe high-resolution scan, small, randomly-spaced microdots of ink ortoner can be detected in areas that appear to be blank or, in otherwords, unprinted, at low-resolution. Detection of these microdotsindicates that a document has been photocopied or laser-printed.

FIG. 2 shows the types of imperfections that can be detected in originaldocuments and in copies of original documents at high-resolution.

FIG. 2 shows the original bill of sale 202 and the duplicate bill ofsale 204 that were shown in FIG. 1. The lower left-hand corners of bothdocuments 202 and 204 are shown as they might appear under20×magnification 206 and 208. At 20×magnification, the original,offset-printed bill of sale might show one or more relatively large andirregularly-shaped imperfections 210 that are not visible to the nakedeye. At 20×magnification, the photocopied or laser-printed bill of sale208 instead contains a number of small, randomly-spaced, generallycircular microdots as, for example, microdot 212. This differencebetween the high-resolution imperfections of original documents andphotocopied or laser-printed documents is the distinguishing featureidentified by the present invention in order to distinguish originaldocuments from photocopied or laser-printed documents. Although a20×magnification has been used in the above example, the optimummagnification, or more specifically the DPI resolution of the scanner,is directly correlated to the resolution of the printing device. Forexample, a document printed with a fairly high resolution device (goodpi or greater) will have toner particles that are smaller (or may besmaller) than the size of the particles created by a 300 DPI laserprinter. As higher resolution printers are used, higher resolutionscanners will need to be employed.

FIG. 3 is a block diagram showing the components of a system 300 onwhich the present invention may be implemented. The system is composedof two main components: an optical scanning device 302 and a computer304. The optical scanning device 302 scans an input document 306 toproduce a digital, two-dimensional matrix representation (not shown) ofthe printed and blank regions of the scanned document 306. This matrixrepresentation of the document is transmitted by the optical scanningdevice 302 through an electrical connection 308 to the computer 304. Thecomputer 304 includes a central processing unit (“CPU”) 310, memory 312,a non-volatile storage device 314, a system controller 316, and internalcommunications busses 318-324 through which data is transferred withinthe computer. The digital matrix representation of a printed documenttransmitted by the optical scanning device 302 passes from the externalconnection 308 to the internal connection 318 and is then routed by thesystem controller 316 either to memory 312 through internal bus 320 orto the non-volatile storage device 318 through bus 324. The digitalrepresentation can then be accessed by the CPU 310 through internalbusses 320 and 322 or through internal busses 324 and 322 and processedby the CPU in order to determine whether the input document 306 is aphotocopied or laser-printed duplicate or whether the document 306 is anoriginal document. The system shown in FIG. 3 is merely one possibleapparatus on which the present invention may be practiced. The presentinvention may alternatively be practiced on a microprocessor connectedto an optical scanning device within some more complex system as, forexample, a photocopier machine. A detector other than an optical scannermight be employed in yet another implementation.

In order to distinguish a photocopied or laser-printed document from anoriginal document, the document is analyzed to determine whether itincludes the above-described microdots. This analysis involves opticallyscanning a portion of the document at both low-resolution andhigh-resolution.

FIG. 4 shows a portion of the example document suitable for analysis.The small rectangular region 402 enclosed within dotted lines of theexample bill of sale 404 will be used in the following discussion todemonstrate the analysis.

FIG. 5 shows a grid representing a low-resolution optical scansuperimposed over a portion of the region shown in FIG. 4. A rectangulargrid 502 has been superimposed over the digits “35” and a portion of theline above the digits. The rectangular grid contains 16 columns labeled0-15 504 in a horizontal direction and 21 rows labeled 0-20 506 in avertical direction. A pair of rectangular coordinates composed of afirst horizontal coordinate corresponding to a column and a secondvertical coordinate corresponding to a row identifies each cell withinthe grid. For example, the upper left-hand starting point of the digit“3” is located within the cell 508 having coordinates (1, 11).

The first, low-resolution optical scan of the document produces atwo-dimensional matrix corresponding to the two-dimensional grid 502shown in FIG. 5. The purpose of the low-resolution scan is to identifythe blank areas that will be subsequently scanned at high resolution todetect microdots within the blank areas. The grid actually produced byoptical scanning may be substantially larger than the grid shown in FIG.5 or may be positioned differently within the document. The grid shownin FIG. 5 represents one possible result of a low-resolution opticalscan of a portion of a printed document. The two-dimensional digitalmatrix produced by the optical scanner has indices corresponding to thehorizontal and vertical coordinates of the rectangular grid 502. Thetwo-dimensional matrix contains a numerical value for each cell of thegrid corresponding to a physical location within the portion of thedocument scanned. Different optical scanners may produce different typesof values depending on the optical reflectivity or some other physicalcharacteristic, of the corresponding location on the surface of thedocument. The present method requires only a single bit for each cell,or, in other words, a two-value designation for each cell. When thevalue in the matrix corresponding to the cell is “0,” the positionwithin the document corresponding to the cell has high reflectivity andis assumed to be blank or non-printed. When the value in the cell is“1,” the reflectivity of the surface of the document corresponding tothe cell is assumed to be low, indicating that it is covered with ink ortoner. Optical scanners may produce some multiple of 8 bits for eachcell in order to represent various gradations of reflectivity thatcompose a gray scale or a color scale. The resent method can bepracticed by assigning a threshold value below which a cell isconsidered to be blank or non-printed and above which a cell isconsidered to be covered with ink or toner, essentially transforming themulti-valued cells into two-valued cells. At low-resolution, as shown inFIG. 5, small imperfections such as microdots that are not visible tothe naked eye are not detected.

A high-resolution scan of the same region scanned at low-resolution asshown in FIG. 5 is shown in FIG. 6. The low-resolution grid issuperimposed on the high-resolution scan in FIG. 6 along with the samenumerical coordinates. The high-resolution scan reveals the digits “3”603 and “5” 604 and the portion of the line above the digits 606 asdetected in the low-resolution scan shown in FIG. 5. The high-resolutionscan also reveals a number of microdots that were not visible in thelow-resolution scan as, for example, microdot 608 within thelow-resolution cell (8, 5) 610. High-resolution grid lines are shown inFIG. 6 for the low-resolution cell 610. The two-dimensional matrix thatwould be produced by the optical scanner to represent thehigh-resolution scan is based on the smaller, high-resolution grid, asshown in FIG. 6 for low-resolution cell 610, with correspondinghigh-resolution coordinates. For example, the high-resolution cell 612at the upper left-hand corner of the low-resolution cell 610 would havehigh-resolution coordinates (40, 20).

The present method compares the low-resolution scan 502 and thehigh-resolution scan 600 in order to detect the microdots revealed inthe high-resolution scan but absent in the low-resolution scan. In thiscomparison, low-resolution cells corresponding to regions of the inputdocument that were determined to be covered by ink or toner or, in otherwords, low-resolution cells having the value “1” are masked out orexcluded from the analysis. Low-resolution cells that are excluded fromthe analysis are indicated in FIG. 6 with cross-hatching, as forexample, the low-resolution cell (12, 11) 614 at the upper right-handcorner of a block of masked low-resolution cells 616 that include theupper horizontal stroke of the digit “5” 604. A conservative exclusioncan be obtained by setting the detection threshold value for thereflectivity of a region of the document at a relatively low value suchthat, for example, if more than one or two high-resolution cells thatcorrespond to a low-resolution cell are covered with ink or toner, thelow-resolution cell will have a value of “1” in the two-dimensionallow-resolution matrix representation. Thus, for example, low-resolutioncell (5, 16) 618 is excluded, as indicated in FIG. 6 with cross-hatchedlines, because the tip of the digit “5” obscures more than onehigh-resolution grid cell within the low-resolution cell. In contrast,low-resolution cell (6, 18) 620 is not excluded because the small regionof the digit “5” covers less than one high-resolution cell. The analysisof the high-resolution matrix representation of the input document willinvolve only those high-resolution matrix cells that correspond to thelow-resolution cells shown in FIG. 6 without cross-hatching.

Although the embodiment explained with reference to FIGS. 3-6 performs alow resolution scan followed by a high resolution scan, it will beunderstood that only a high resolution scan is required. However,scanning the entire document at high resolution will generally requiremore time than scanning the entire document at low resolution and thenscanning only a portion of the document at high resolution.

FIG. 7 shows idealized microdots positioned within a portion of ahigh-resolution scan grid. The embodiment of the present inventiondescribed below assumes that the optical scanning device employed toproduce the low-resolution and high-resolution scans provides adjustableor selectable resolution. In this case, it is desirable to adjust theresolution for the high-resolution scan so that the dimension of a gridcell roughly corresponds to the average diameter of a microdot. If anoptical scanner does not provide such adjustable resolution,mathematical techniques can be employed to transform a two-dimensionalmatrix representation produced by the optical scanner to an appropriateresolution. Alternatively, different embodiments of the present methodmay be used for detecting microdots at resolutions lower or higher thanthis desirable resolution.

Circle 702 in FIG. 7 represents a microdot centered within thehigh-resolution cell 704, illustrating the approximate correspondencebetween the average diameter of a microdot and the dimension of thehigh-resolution cell. Microdot 706 is centered at the intersection offour high-resolution cells 708. Each of the four high-resolution cellsthat contain portions of microdot 706, 710-716 contain exactly 25% ofthe idealized circular microdot 706. Cell 704 contains essentially 100%of microdot 702. As will be illustrated below, it is desirable to setthe reflectivity threshold for the high-resolution scan such that a cellcontaining more than 25% of a microdot will have a resulting scan valueof “1” while a cell containing 25% or less of a microdot will have aresulting scan value of “0.” Thus, cell 704 will have a scan value of“1” and the four cells 710-716 will each have a scan value of “0.” Thethreshold is set in this way in order to ensure that, at most, only twocells that share a common side and have a scan value of “1” willcorrespond to a single microdot. In other words, with the threshold setto greater than 25%, a single microdot can, at most, cause two cellssharing a common side to have a value of “1.” In the special case of amicrodot centered at the intersection of four cells, like microdot 706,the microdot will produce no cells having a value of “1” and will thusnot be detected by the high-resolution scan. In general, the centers ofmicrodots will not coincide with the intersection of four cells, andloss of detection of the small number of microdots with centerscoinciding with four-cell intersections does not impact the overalldetermination of whether microdots are present on the input document.Microdots 718-726 have centers slightly dislocated from intersectionpoints of four cells in order to illustrate the fact that a microdot cancause at most two adjoining cells to have the value of “1.” In the caseof microdot 718, the center has been slightly translated along a celledge to the left from the intersection point of the four cells. In thiscase, cell 728 and cell 730 will each contain greater than 25% of thearea of microdot 718 and will thus each have a scan value of “1.” Cells732 and 734 will have less than 25% of the area of microdot 718 and willthus have scan values of “0.” Thus, microdot 718 is detected in thehigh-resolution scan by two adjacent high-resolution cells 728 and 730having scan values of “1.” Microdot 720 is translated vertically fromthe intersection point of four cells resulting in cells 736 and 738 eachcontaining greater than 25% of the total area of a microdot and thushaving a scan value of “1” and high-resolution cells 740 and 742 eachhaving less than 25% of the area of a microdot and thus having scanvalues of “0.” Microdots 722, 724, and 726 show progressive translationof the center of the microdot in a diagonal direction from theintersection points of four high-resolution cells. In this case, thecell into which the center of the microdot is translated 744, 746, and748, respectively, will contain greater than 25% of the total area ofthe microdot and the remaining three high-resolution cells that containportions of the microdot will contain less than 25% of the microdot.Thus, when the dimension of a high-resolution cell roughly correspondsto the diameter of a microdot, a given microdot will generally causeonly one high-resolution cell to have a scan value of “1” and at most,will cause two adjacent high-resolution cells to have scan values of“1.” This matching of resolution to microdot size greatly simplifies thedetection algorithm to be described below. In essence, microdotdetection amounts to finding single high-resolution cells, or pairs ofhigh-resolution cells that share a common side, with scan values of “1”surrounded on all sides by high-resolution cells having scan values of“0.”

FIG. 8 is a flow control diagram for one embodiment of the presentmethod. In step 802, a portion of a document is scanned atlow-resolution, as illustrated in FIG. 5. In step 804, the methoddetermines the ratio of low-resolution cells having scan value less thansome low threshold value to the total number of low-resolution cellsincluded in the scan. In the case of a matrix having binary values, thisis the ratio of low-resolution cells having the value “0” divided by thetotal number of low-resolution cells in the scan. In step 806, themethod compares this ratio to a threshold value. When the calculatedratio is less than a threshold value, the method determines that thereis not enough white or blank space within the document for analyzing thedocument for the presence of microdots. In this case, the methodreturns, in step 808, an indication that it cannot determine whether thedocument in question is an original document or a photocopy or aduplicate. Alternatively, the present method could try additionallow-resolution scans of different portions of the document beforeconcluding that the document contains insufficient white or blank space.If, on the other hand, the calculated ratio exceeds the threshold value,then control flows to step 810. In step 810, the method scans the sameportion of the document scanned in step 802 at high-resolution.

In step 812, the method employs an alignment procedure to ensure thatthe two scans are properly aligned with each other, or, in other words,ensures that the coordinates or indices of the matrix representation ofthe high-resolution scan are fixed multiples of the indices orcoordinates of the matrix representation of the low-resolution scans andthat the upper, left-hand corner of the first cell in thehigh-resolution matrix coincides with the upper left-hand corner of thefirst cell in the low-resolution matrix. An optical scanner mayautomatically ensure such alignment, in which case step 812 is notnecessary. If alignment is necessary, it can be relatively simplyachieved by calculating the center of density of the low-resolution andhigh-resolution scans, superimposing the two scans so that the centersof density coincide, and then translating and rotating one scan withrespect to the other in order to maximize a computed correspondencebetween cell values of the two scans.

Once the two scans are aligned, the method, in step 814, calls aPhotocopy routine to analyze the low-resolution and high-resolutionmatrix representations of the scans in order to detect the presence ofmicrodots. If the result of the Photocopy routine indicates thatmicrodots are present, as determined in step 816, then the methodreturns an indication that the analyzed document is a photocopied orlaser-printed duplicate in step 818. Otherwise, the method returns anindication that the analyzed document is an original document in step820.

A pseudo-code implementation of the routine photocopy is shown below.FIG. 9 illustrates operation of the routine Photocopy. In the text thatfollows the routine Photocopy, the routine will be described withrelation to FIG. 9.

1 #define THRESHOLD = 0.01; 2 #define MASKED (arg1, arg2)low_res[arg1/MAG, arg2/MAG] 3 4 int photocopy (int HD, int VD, int MAG,int *low_res, int *high_res) 5 { 6 int i; 7 int j; 8 int one = FALSE; 9int num_masked = 0; 10 int num_microdots = 0; 11 doublemicrodot_density; 12 13 for (j = 2; j < VD − 2; j++) 14 { 15 for(i = 2;i < HD − 2; i++) 16 { 17 if (MASKED(i,j) num_masked++; 18 elseif(high_res[ij]) 19 { 20 one = FALSE; 21 if (high_res[i−1,j] ||MASKED(i−1,j) || 22 high_res[i,j−1] || MASKED(i,j−1) || 23 MASKED(i+1,j) || MASKED(i,j+1)) continue; 24 if (high_res[i+1,j] 25 { 26if(high_res[i+1,j−1] || high_res[i +2,j] || high_res[i+1,j+1]) continue;27 if(MASKED(i+1,j−1) || MASKED(i+2,j) || MASKED(i+1,j+1)) continue; 28one = TRUE; 29 } 30 if (high_res[i,j+1]) 31 { 32 if (one) continue; 33if (high_res[i−1,j+1] || high_res[i,j+2] || high_res[i+1,j+1]) continue;34 if (MASKED(i−1,j+1) || MASKED(i,j+2) || MASKED(i+1,j+1)) continue; 35} 36 num_microdots++; 37 } 38 } 39 } 40 microdot_density = num_microdots/ (((HD-2) * (VD-2)) - num_masked); 41 if (microdot_density > THRESHOLD)return TRUE; 42 else return FALSE; 43 }

In line 1 of the routine Photocopy, the constant THRESHOLD is defined tobe 0.01. At the end of the routine Photocopy, the density of microdotsdetected in a matrix representation of a scan is calculated and comparedto this threshold. If the microdot density exceeds this threshold, thenthe routine Photocopy will return an indication that microdots have beendetected. The value for this threshold must be determined byexperimentation for a given optical scanner and for given low and highresolutions. Line 2 of the routine Photocopy defines a macro calledMASKED. The macro takes two arguments “arg1” and “arg2” which representthe rectangular coordinates of a high-resolution matrix cell. The macroreturns the value of the low-resolution matrix cell that corner pointsto this high-resolution matrix cell. Thus, the value returned by thismacro is “1” when the low-resolution cell has a scan value of “1” and“0” when the low-resolution cell has a scan value of “0.” The value “0”is equivalent to the Boolean value FALSE and the value “1” is equivalentto the Boolean value TRUE. Thus, applying this macro to the coordinatesof a high-resolution cell returns a Boolean value indicating whether thehigh-resolution cell should or should not be considered in the microdotanalysis, or, equivalently, whether or not the low-resolution matrixcell corresponding to the high-resolution matrix cell has been maskedout of the analysis.

As can be seen in line 4, the routine Photocopy takes five arguments:HD, VD, MAG, LOW_RES, and HIGH_RES. HD is the horizontal dimension ofthe high-resolution matrix and VD is the vertical dimension of thehigh-resolution matrix. In the high-resolution matrix 900 shown in FIG.9, HD would have the value 20 and VD would also have the value 20,corresponding to the 20 columns 902 and 20 rows 904 of thehigh-resolution matrix. The argument MAG is the magnification factorbetween the low-resolution and high-resolution scans. Taking thelow-resolution and the high-resolution scans of FIGS. 5 and 6 as anexample, the magnification factor would be 5 since there are fivehigh-resolution cells in a horizontal direction and five high-resolutioncells in a vertical dimension along the horizontal and vertical edges ofa single low-resolution cell. The argument low_res is a pointer to thetwo-dimensional matrix representation of a low-resolution scan and theargument high_res is a pointer to a two-dimensional matrixrepresentation of a high-resolution scan. In both cases, each cell ofthe matrix is represented by an integer value. Blank or white space isindicated by a value of “0,” and printed or toner-impregnated areas areindicated by a value of “1.”

The six local variables used in the routine Photocopy are declared inlines 6-11. The integer variables “i” and “j” correspond to thehorizontal and vertical rectangular coordinates of matrix cells, asindicated by the directional arrows 906 and 908 of FIG. 9. The integervariable “num_masked” is used in Photocopy to accumulate the number ofmasked hioh-resolution cells. The variable “num_microdots” is used toaccumulate the number of microdots detected by Photocopy. The variable“microdot_density” is used to hold a calculated value of the number ofmicrodots detected per the number of unmasked high-resolution cellsanalyzed.

The bulk of the analysis, in which the variables “num_masked” and“num_microdots” accumulate the number of masked cells and the number ofdetected microdots, respectively, occurs within two nested “for” loopson lines 13-39. The effect of the nested “for” loops is to analyze, oneby one, cells within the high-resolution scan. The analysis proceeds asindicated by the dotted arrows 910 and 912 in FIG. 9. Starting with cell(2, 2), each successive cell in the horizontal direction is consideredup through cell (17, 2). Then analysis begins on the next row at cell(2, 3) and again proceeds one-by-one in a horizontal direction. Thecells within two vertical and two horizontal margins along the edgeetches of the high-resolution matrix are not included in the analysis.

The code included in lines 17-36 is applied, in succession, to each ofthe analyzed cells. Within this code, the cell being analyzed isdesignated as “high_res [i, j].” On line 17, the macro MASKED is appliedto cell (i, j) to determine whether the currently analyzed cell shouldbe included in the microdot analysis. If the macro MASKED returns avalue of “1,” or TRUE, the variable “num_masked” is incremented toindicate detection of another masked cell and the remaining lines of thenested “for” loops 18-36 are skipped for cell (i, j). If cell (i, j) isnot masked, then, in the “if” statement on line 18, Photocopy determineswhether the cell (i, j) indicates detection of ink or toner. If not,cell (i, j) cannot represent a microdot, and the remaining lines 17-36of the nested “for” loops are skipped for cell (i, j). The variable“one” is set to FALSE on line 20 to indicate that no adjoining cellswith the value of “1” have yet been detected for cell (i, j). In the“if” statement beginning on line 21 and extending through line 23,adjacent neighbors of cell (i, j) are examined for certaincharacteristics that will exclude cell (i, j) from further analysis. Anexample cell (i, j) 914 with i=6 and j=7 is shown in FIG. 9. Thisexample cell has four adjacent neighboring cells, a north neighbor 916,an east neighbor 918, a south neighbor 920 and a west neighbor 922. Ifany of these adjacent neighboring cells are masked or if the north orwest neighbor have values of “1,” cell (i, j) will not be furtherconsidered. High-resolution cells adjacent to masked cells are notconsidered because it is possible that they represent extensions offeatures visible to the naked eye and detected in the low-resolutionscan. If either the north or west adjacent neighbor has a value of “1,”then cell (i, j) has already been considered in the analysis in aprevious iteration of the nested “for” loops and need not bereconsidered. Thus, if cell (i, j) does not have masked neighbors andwas not previously considered, the analysis continues at line 24.Otherwise, lines 24-36 are skipped and Photocopy continues with theanalysis of a subsequent cell.

At this point in the analysis, cell (i, j) has been determined to be acandidate microdot. The remaining code in lines 24-37 is directed todetermining whether this candidate cell has more than one adjacentneighboring cell that also has a value of “1.” If so, as discussed aboveand illustrated in FIG. 7, then cell (i, j) cannot be considered to be amicrodot because it will have dimensions greater than a microdot. If thecandidate cell (i, j) has no adjacent neighbor with scan value of “1” orhas only one adjacent neighbor with the scan value of “1,” hen cell (i,j) is assumed by Photocopy to represent a detected microdot. If the eastneighbor has a value of “1” and the east neighbor has no masked adjacentneighbors and has no adjacent neighbors with a value of “1,” thenvariable “one” is set to TRUE on line 28. If the east neighbor has avalue of “0,” then the variable “one” remains FALSE, as initialized inline 20.

A similar analysis is conducted in lines 30-35 for the south neighbor ofthe candidate cell (i, j). If the south neighbor has a value of “1,” andthe east neighbor was detected to have a value of “1,” then candidatecell (i, j) has two adjacent neighbors with the value of “1” and cannotbe considered as a microdot. Thus, further analysis for cell (i, j) isskipped. If the east neighbor does not have a value of “1,” and thesouth neighbor does have a value of “1” but also has an additionaladjacent neighbor that is either masked or has a value of “1,” thencandidate cell (i,j) cannot be considered to be a microdot. If executionreaches line 36, then candidate cell (i, j) has been determined to be amicrodot and the variable “num_microdots” is incremented to indicatethat another microdot has been detected. Once all the possible candidatecells have been analyzed within the nested four loops, the microdotdensity is calculated on line 40 as the sum of the number of microdotsdetected divided by the number of unmasked cells within thehigh-resolution matrix. If the microdot density exceeds the thresholdvalue, then, on line 41, Photocopy returns TRUE, indicating thatmicrodots have been detected. Otherwise, Photocopy returns a value ofFALSE on line 42.

Referring to FIG. 9. cells 914, 924, and 926 would each be determined tobe a separate microdot by the analysis of the routine Photocopy. Cells928 and 930 would be together determined to be a single microdot andlikewise cells 932 and 934 would be together determined to be a singlemicrodot. The two blocks of three contiguous cells 936 and 938 would notbe determined to be microdots by the Photocopy.

Thus, the above-described present method involves comparison of alow-resolution and high-resolution scan of the same portion of adocument with appropriately set thresholds for each scan. Thelow-resolution scan is used to mask out cells of the high-resolutionscan from analysis so that features appearing visible to the naked eyeare not considered in the microdot analysis. The unmasked regions of thehigh-resolution scan are then analyzed to detect the presence of singlecells or pairs of cells having the value of “1” surrounded by cellshaving the value “0,” or, in other words, discrete spots within thehigh-resolution scan that were not detected at low-resolution. Thepresence of such spots indicates the presence on the surface of thedocument of microdots, characteristic of photocopied or laser-printeddocuments.

Although the present invention has been described in terms of oneembodiment, it is not intended that the invention be limited to thisembodiment. Modification within the spirit of the invention will beapparent to those skilled in the art. For example, the method can beimplemented either in a stand-alone system as shown in FIG. 3, or on amicroprocessor and scanning apparatus embedded within a more complexsystem such as a photocopier. A variable-resolution scanner can beemployed in order to produce a high-resolution scan with grid dimensionsapproximately equal to the average diameter of a microdot or,conversely, a scanner having fixed resolutions can be used and theresulting matrix representation transform mathematically to produce anequivalent resolution with grid dimensions equal to the average diameterof a microdot. Alternatively, a more complex implementation of theroutine Photocopy may be employed to analyze a high-resolution scanwhere the diameter of a microdot exceeds the grid dimensions. In such acomplex implementation, the matrix representation of the scan documentwould need to be examined for discrete spots comprising multiple cellshaving scan values of “1” surrounded by regions of cells having scanvalues of “0.” Alternative methods for detecting the presence of ink ortoner on the surface of a document other than optical scanning could beemployed to generate the matrix representation of a portion of thesurface of the document. Many different implementations of the routinePhotocopy are possible. The scope of the present invention is defined bythe claims that follow.

What is claimed is:
 1. A method for determining whether a document hasbeen created by photocopying or laser printing, comprising detectingrandomly-spaced microdots produced during photocopying and laserprinting on the surface of a document, and, in response to detecting themicrodots, determining that the document has been created byphotocopying or laser printing.
 2. The method of claim 1 whereindetecting microdots produced during photocopying and laser printingfurther comprises: counting the number of microdots within an area ofthe document; comparing the number of microdots counted to a thresholdvalue; when the number of microdots counted exceeds the threshold value,determining that microdots are present on the surface of the document;and when the number of microdots counted is equal to or less than thethreshold value, determining that microdots are not present on thesurface of the document.
 3. The method of claim 2 wherein counting thenumber of microdots within an area of the document is accomplished by:employing an optical scanner to measure the reflectivity of the surfaceof the area of the document, the result of measurement of thereflectivity of the surface of the document output as a scan; anddetecting microdots in the scan.
 4. A method for determining whether adocument has been created by photocopying or laser printing, comprisingdetecting microdots produced during photocopying and laser printing onthe surface of a document by: counting the number of microdots within anarea of the document by employing an optical scanner to measure thereflectivity of the surface of the area of the document, the result ofmeasurement of the reflectivity of the surface of the document beingoutput as a scan, and detecting microdots in the scan by the steps of:employing an optical scanner to measure the reflectivity of the surfaceof the area of the document at low resolution to produce alow-resolution scan in which microdots are not detectable; employing anoptical scanner to measure the reflectivity of the surface of the areaof the document at high resolution to produce a high-resolution scan inwhich microdots are detectable; and detecting microdots in that portionof the high-resolution scan that corresponds to those areas of thelow-resolution scan in which the measured reflectivity does not indicatethe presence of ink, toner, and other printing substances; comparing thenumber of microdots counted to a threshold value; when the number ofmicrodots counted exceeds the threshold value, determining thatmicrodots are present on the surface of the document; and when thenumber of microdots counted is equal to or less than the thresholdvalue, determining that microdots are not present on the surface of thedocument, and, in response to detecting the microdots, determining thatthe document has been created by photocopying or laser printing; and inresponse to determining that microdots are present on the surface of adocument, determining that the document has been created by photocopyingor laser printing.
 5. The method of claim 4 wherein detecting microdotsfurther comprises detecting small regions within the high-resolutionscan that differ in reflectivity from the reflectivity of unprintedregions of the document.
 6. The method of claim 5 wherein thehigh-resolution scan comprises a high-resolution matrix having cellsthat each contain a value that indicates the reflectivity at acorresponding position on the surface of the document, the area of thesurface of the document corresponding to each cell of thehigh-resolution matrix approximately equal to the area of a microdot. 7.The method of claim 6 wherein one high-resolution matrix cell having avalue indicating a reflectivity corresponding to ink, toner, or otherprinting substances adjoining on all sides high-resolution matrix cellshaving a value indicating a reflectivity corresponding to an unprintedsurface of the document is detected as a microdot.
 8. The method ofclaim 6 wherein two high-resolution matrix cells, adjoined along acommon side, having values indicating a reflectivity corresponding toink, toner, or other printing, substances adjoining on all sides otherthan the common side high-resolution matrix cells having a valueindicating a reflectivity corresponding to an unprinted surface of thedocument are together detected as a microdot.
 9. A method fordetermining whether a document on which one or more images have beencreated has been printed by photocopying or laser printing, comprising:scanning a portion the document at low resolution to identify blankareas of the document on which the one or more of the images have notbeen created; scanning the blank areas of the document at highresolution to detect microdots; and and in response to detecting themicrodots, determining that the document has been created byphotocopying or laser printing.
 10. The method of claim 9 whereinscanning the portion of the document at low resolution produces alow-resolution matrix having cells, each cell containing a value thatindicates presence or absence of substances employed in photocopying andlaser printing at a position on the document corresponding to the celland wherein scanning the blank areas of the document at high resolutionproduces a high-resolution matrix having cells, each cell containing avalue that indicates presence or absence of substances employed inphotocopying and laser printing at a position on the documentcorresponding to the cell.
 11. The method of claim 10 wherein an area ofthe document corresponding to one high-resolution matrix cell isapproximately equal to the area of a microdot.
 12. The method of claim11 wherein detecting microdots produced during photocopying and laserprinting further comprises detecting high-resolution matrix cells thathave a value that indicates presence of toner or other substancesemployed in photocopying and laser printing and that share common sideswith adjoining high-resolution matrix cells that all have values thatindicate absence of toner or other substances employed in photocopyingand laser printing.
 13. The method of claim 12 wherein detectingmicrodots further comprises detecting pairs of high-resolution matrixcells that share a common side, that have lower reflective values, andthat adjoin on all sides other than the common shared sidehigh-resolution matrix cells that all have lower reflectivity values.14. A method for determining whether a document was produced byphotocopying or laser-printing, the method comprising: scanning an areaof the document for the presence of substances that differ inreflectivity from a background reflectivity of the document at aresolution sufficient to detect microdots characteristic of photocopiedor laser-printed documents; analyzing an area of the scanned area tocount the number of microdots within the analyzed area; computing avalue based on the number of microdots counted; comparing the computedvalue to a threshold value; when the computed value exceeds thethreshold value, returning an indication that the document was producedby photocopying or laser-printing; and when the computed value is equalto or less than the threshold value, returning an indication that thedocument was not produced by photocopying or laser-printing.
 15. Themethod of claim 14 wherein the computed value is the number of microdotswithin the analyzed area divided by the size of the analyzed area andthe threshold value is a threshold density of microdots.
 16. The methodof claim 14 wherein scanning for the presence of substances isaccomplished by employing an optical scanner that measures thereflectivity of the surface of the document.
 17. A method fordetermining whether a document was produced by photocopying orlaser-printing, the method comprising: scanning an area of the documentfor the presence of substances that differ in reflectivity from abackground reflectivity of the document at a resolution sufficient todetect microdots characteristic of photocopied or laser printeddocuments at a lower resolution at which microdots are not detectable;analyzing an area of the scanned area at high resolution thatcorresponds to those areas of the low resolution scan on which thepresence of a substance was not detected to count the number ofmicrodots within the analyzed area; computing a value based on thenumber of microdots counted; comparing the computed value to a thresholdvalue; when the computed value exceeds the threshold value, returning anindication that the document was produced by photocopying orlaser-printing; and when the computed value is equal to or less than thethreshold value, returning an indication that the document was notproduced by photocopying or laser printing.
 18. The method of claim 17wherein scanning the area of the document for the presence of substancesat low resolution produces a low-resolution matrix having cells thateach contain a value that indicates that the presence or absence of asubstance at corresponding position on the surface of the document andwherein scanning the area of the document for the presence of substancesat high resolution produces a high-resolution matrix having cells thateach contain a value that indicates that the presence or absence of asubstance at corresponding position on the surface of the document. 19.The method of claim 18 wherein the resolution of the high-resolutionscan is chosen so that a dimension of area of the surface of thedocument corresponding to a cell of the high-resolution matrix isapproximately equal to the average diameter of a microdot and whereinanalyzing that portion of the area at high resolution that correspondsto those areas of the low resolution scan on which the presence of asubstance was not detected further includes: initializing a count ofdetected microdots to 0; successively choosing each unanalyzed cellwithin a portion of cells of the high-resolution matrix for analysis;and when the value of the chosen cell indicates presence of a substanceand the value of the cell of the low-resolution matrix that correspondsto the chosen cell indicates absence of a substance and the chosen cellhas at most one adjacent high-resolution cell with a value indicatingpresence of a substance and that adjacent has no additional adjacenthigh-resolution cells with a value indicating presence of a substance,determining that the chosen cell and the at most one adjacent cellcomprise an indication of a microdot and incrementing the count ofdetected microdots by one.
 20. A method for detecting small discreetmicrodots produced during photocopying and laser printing on the surfaceof a document, the method comprising: scanning a portion the document atlow resolution to produce a low-resolution matrix, each cell of thelow-resolution matrix corresponding to a discrete low-resolution regionon the surface of the document and having a value that indicates whethera photocopying or laser printing substance was detected on that regionof the document, the discrete low-resolution region on the surfacesufficiently large and the threshold for substance detectionsufficiently high, that a presence of a number of microdots within adiscrete low-resolution region otherwise free of photocopying or laserprinting substances is not detected; scanning the portion the documentat high resolution to produce a high-resolution matrix, each cell of thehigh-resolution matrix corresponding to a discrete high-resolutionregion on the surface of the document smaller than a discretelow-resolution region corresponding to a low-resolution matrix cell andhaving a value that indicates whether a photocopying or laser printingsubstance was detected on the high-resolution region of the document, ahigh-resolution region sufficiently small, and the threshold forsubstance detection sufficiently low, that a presence of a number ofmicrodots within a discrete high-resolution region otherwise free ofphotocopying or laser printing substances is detected; and comparing thelow-resolution matrix to the high-resolution matrix to detectindications of microdots.
 21. The method of claim 20 wherein a dimensionof the high-resolution region is approximately equal to the averagediameter of a microdot.
 22. The method of claim 21 wherein thelow-resolution and high-resolution matrices are aligned so that theupper left-hand portion of the portion of the document corresponding tothe first cell of the low-resolution matrix corresponds to first elementof the high-resolution matrix and so that the portion of the documentrepresented by the low-resolution matrix is equal to the portion of thedocument represented by the high-resolution matrix, each cell of thelow-resolution matrix corresponding to a number of cells of thehigh-resolution matrix.
 23. The method of claim 22 wherein comparing thelow-resolution matrix to the high-resolution matrix further comprises:choosing as a candidate cell each successive, unanalyzed cell of acontiguous set of cells within the high-resolution matrix that has avalue indicating detection of a photocopying or laser printing substanceand that corresponds to a low-resolution matrix cell that has a valueindicating that a photocopying or laser printing substance was notdetected; and for each candidate cell, when the candidate cell has oneadjoining, unanalyzed cell that has a value indicating detection of aphotocopying or laser printing substance and that corresponds to alow-resolution matrix cell that has a value indicating that aphotocopying or laser printing substance was not detected and when theadjoining cell has no adjoining cells in addition to the candidate cellthat have a value indicating detection of a photocopying or laserprinting substance, analyzing the candidate cell and the one adjoiningcell together as a candidate microdot; and when the candidate cell hasno adjoining cells that have a value indicating detection of aphotocopying or laser printing substance, analyzing the candidate cellas a candidate microdot.
 24. The method of claim 23 wherein analysis ofa candidate microdot comprises: determining whether any cell adjoiningthe candidate microdot correspond to a low-resolution matrix cell thathas a value indicating detection of a photocopying or laser printingsubstance; when no cell adjoining the candidate microdot corresponds toa low-resolution matrix cell that has a value indicating detection of aphotocopying or laser printing substance, determining that the candidatemicrodot is an indication of a microdot; and when a cell adjoining thecandidate microdot corresponds to a low-resolution matrix cell that hasa value indicating detection of a photocopying or laser printingsubstance, determining that the candidate microdot is not an indicationof a microdot.
 25. The method of claim 24, further including: countingthe indications of a microdot within the high-resolution matrix;dividing the count of the indications of a microdot within thehigh-resolution matrix by the area of the portion of the documentscanned to computer a microdot density; and comparing the microdotdensity to a threshold value to determine whether the document containsmicrodots.
 26. The method of claim 20 wherein scanning for the presenceof photocopying or laser printing substances is accomplished byemploying an optical scanner that measures the reflectivity of thesurface of the document.
 27. The method of claim 17 wherein the computedvalue is the number of microdots within the analyzed area divided by thesize of the analyzed area and the threshold value is a threshold densityof microdots.
 28. The method of claim 17 wherein scanning for thepresence of substances is accomplished by employing an optical scannerthat measures the reflectivity of the surface of the document.
 29. Amethod for determining whether a document was produced by photocopyingor laser-printing, the method comprising: scanning a blank area of adocument; analyzing said blank area a resolution sufficient to detectmicrodots characteristic of photocopying and laser printing to determinewhether such microdots are present in the blank area; determining thatthe document was produced by photocopying if a value representative ofthe number of microdots present in the blank area is greater than athreshold value.
 30. The method of claim 29 wherein the threshold isrepresentative of the number of microdots per unit area and wherein thevalue representative of the number of microdots detected is equal to thenumber of microdots detected divided by the area of the blank area. 31.The method of claim 29 wherein scanning of a blank area of a document isaccomplished by employing an optical scanner that measures thereflectivity of the surface of the document.